1. Field of the Invention
The present invention relates to a hydrogel suitable as a carrier for microorganism for waste water treatment, a bioreactor and the like and, also, to a process for producing the same.
2. Description of the Prior Art
Polymer gels have been vigorously studied as carriers for biocatalysts, water-retaining agents, cooling agents, replacements for biogels such as eye, skin and joints, controlled release material for medicines and base material for actuator.
Known polymer hydrogels comprise agar, alginates, carrageenan, polyacrylamide, polyvinyl alcohol (hereinafter referred to as "PVA") and photo-curing resins. Among these, PVA-based gels, having high mechanical strength and excellent hydrophilic properties (water-containing capability, inhabitability for microorganisms and the like), have been actively studied and, in particular, have attracted attention as carriers for microorganisms.
It is necessary that hydrogels used as carriers for microorganisms and bioreactors have high retaining capability for microorganisms and water and high capability for trapping microorganisms and other fine matter. Further improvement of these capabilities have been studied from various angles.
For example, Japanese Patent Application Laid-open No. 41516/1995 discloses a PVA gel obtained by contacting an aqueous solution containing PVA and sodium alginate with an aqueous calcium chloride solution or the like to solidify the sodium alginate present at least on the surface of the PVA solution to shape the mixture and then gelling the shaped matter by repeated cycles of freezing and thawing or by contacting with a coagulating liquid for PVA. However, the gel obtained by contacting with the coagulating liquid for PVA has low capability for trapping microorganisms (see FIG. 4), because of its smooth surface. The gel obtained by repeated cycles of freezing and thawing has on its surface projections and recesses, which however do not penetrate into the inside of the gel, but also has a dense layer near the surface, so that microorganisms cannot get into the inside and can live only on the surface of the gel (see FIG. 5).
Japanese Patent Application Laid-open No. 17904/1988 discloses a huge granular porous PVA obtained by dispersing a mixed aqueous solution of PVA and a spontaneous gelation accelerating agent such as sodium chloride in an organic solvent such as toluene, to form spherical granules, and then allowing them to gel by themselves. By this process, the PVA precipitates from the aqueous mixed solution and causes phase separation, so that the precipitates have heterogeneous structure and hence give huge porous PVA granules. However, this process can only produce a gel having a smooth surface, like one obtained by contact with a PVA coagulating liquid (FIG. 4).
Japanese Patent Application Laid-open No.276488/1988 discloses a process which comprises adding sodium hydrogencarbonate or the like to an aqueous PVA solution to generate fine bubbles and then subjecting the mixture to freezing and thawing treatment, to effect gelation. The obtained gel, however, has a smooth surface and hence has low capability for trapping microorganisms. Furthermore, the gel has large pores (size: at least 100 .mu.m) formed from bubbles, so that microorganisms, having easily invaded into the inside, cannot be retained over a long period of time.
Japanese Patent Application Laid-open No. 251190/1995 discloses a gel having a structure where short fibers are planted on the surface like mustache. The gel is obtained by adding dropwise a mixed solution of PVA and sodium alginate to an aqueous calcium chloride solution having dispersed short fibers. However, adhering short fibers by such a process results in deterioration in gel properties. Furthermore, presence of mustache-like fibers tends to make the gel granules entangle with each other, so that the granules are readily damaged. In addition, no net-like structure with a multiplicity of fibers can be formed on the surface, which leads to insufficient improvement in capability for trapping microorganisms.